The invention relates to a tape cassette, and more particularly, to a tape cassette utilizing a magnetic tape having a magnetizable surface formed by a thin metal film in which an arrangement is made to prevent the tape from being charged.
A variety of new kind of magnetic tapes, commonly referred to as metallic tapes, are proposed for use in a magnetic recording and reproducing apparatus such as a tape recorder, a video tape recorder (VTR) or the like in order to achieve high recording density. As one example, a thin metal film may be formed on a plastic film base by a sputtering of cobalt-chromium (Co-Cr) for use as a magetizable surface of magnetic tape. Such tape is already in use. Similarly, a magnetic tape having a magnetizable surface formed by an evaporated magnetic film of a cobalt alloy is also appearing on the market.
Since such a thin metal film which forms the magnetizable surface of such magnetic tape represents a good conductor, a triboelectricity which is developed as the magnetic tape is being fed by the apparatus does not remain in the place of their original occurrence, but will be distributed along the full length of the tape. If the magnetic tape which is electrostatically charged in this manner obtains a high electric potential, there will be produced an electric discharge between the magnetizable surface and any metallic member which is located adjacent to the tape, causing a crackling.
It is generally recognized that a tape cassette which contains a length of magnetic tape comprises a pair of upper and lower cassette halves which are molded from a plastic material with the opposite ends of the tape being secured to left- and right-hand hubs and wound thereon. The front side of the cassette is formed with slots or windows into which a magnetic head, a pinch roller or the like may move and along which the tape runs in contact with tape guide pins, tape guide rollers or tape pads while being maintained in taut condition. A path for the discharge of the electrostatic charge which is produced by the triboelectric effect of the magnetic tape is located on the part of the recording and reproducing apparatus on which the cassette is mounted, and may include the pinch roller, the capstan and the magnetic head. However, the pinch roller usually comprises an insulating material such as rubber, and hence cannot form a discharge path for the charge. Also, the capstan is disposed in abutment against the nonmagnetic surface of the magnetic tape, again preventing a discharge of the charge to the capstan. A magnetic head which comprises a ferrite material has no electrical conductivity, while a magnetic head which comprises a permalloy material is coated by an electrically nonconductive resin, both preventing a discharge of the electrostatic charge to the head itself. However, a magnetic head is housed in a shielded metal casing which is usually provided with an integral tape guide. Hence, the electrostatic charge on the tape will be discharged to the tape guide.
The electrostatic charge can be discharged to the tape guide without any problem whatsoever if a reliable contact is maintained between the tape and the guide. However, a bouncing in the movement of the tape as it is being fed may cause an irregular and repeated contact between the tape and the tape guide, thereby giving rise to a crackling during the discharge process which may be picked up by the head and reproduced as noise.